Menstrual cycle tracking apps are the next big bonanza for period science; by collecting data (consensually) from thousands, even millions, of people, these apps can back up or even contribute to scientific research about many aspects of female health, from STD symptoms, to the phenomenon of premenstrual syndrome. And that extends to exercise and its interactions with the different stages of the menstrual cycle. The bad news, if you're lazy (like me), is that there's no "forbidden" time to exercise your cycle, but different phases of your cycle do have different impacts on your physical activity. Bustle talked to the senior designer behind high-rated period app Clue, Caroline Hardy, who's also a Crossfit athlete, about what it takes to train in sync with your cycle, and how your flo influences different aspects of your workout.

Exercise, Endurance, And Your Cycle

Clue's research scientists have been looking at the connections between exercise and the menstrual cycle from various different angles, referencing past studies. "Physical endurance is influenced by estrogen and progesterone because these hormones regulate body temperature, metabolism and heart function," they tell Bustle. "Our hormones also change the elasticity of our ligaments."

To understand the differences in the body's responses to exercise, you need to familiarize yourself with the stages of menstrual cycles. The follicular phase, which extends from the end of your period to ovulation (day 7 through 14 or so of a 28-day cycle), is the point at which your body gets ready to release an egg and grows the endometrium, the lining of the uterus, to prepare for pregnancy. This phase tends to coincide with higher estrogen levels. Ovulation occurs in the middle of your cycle, from day 14 to around day 21. The luteal phase, which occurs after the body releases the egg during ovulation, lasts from ovulation until your period (days 21 through 28-ish), increases progesterone and then decreases both hormones suddenly in preparation for bleeding. "Several studies have looked at differences in responses to strength training in the follicular phase, versus training in the luteal phase," says Hardy. "Some research has found that strength training during the follicular phase resulted in higher increases in muscle strength compared to training in the luteal phase. If you start paying attention to your cycle phases, you may find your strength training pays off the most in your follicular phase."

It's not just strength, either. Hormonal shifts around periods may also raise the risk of injuries, says Hardy. "A recent meta-review of studies looked at how hormonal changes may impact tendon laxity and risk of tendon injury. It found the risk was highest in the days leading up to ovulation, when estrogen is high," she says. "The luteal phase was associated with the lowest risk" of injury. Scientists need to confirm the link, but she recommends doing a lot of warm-ups and cool-downs when you're ovulating, and to be careful about stretching.

The luteal phase, Hardy says, is demanding on your body (you may recognize this as your PMS phase), so your energy and endurance levels might not be up to the same levels they were in your follicular phase. "Your body is preparing for a potential pregnancy, should an egg have been fertilized at ovulation," she says. "As a result, you may find that you don’t have as much endurance during your luteal phase. You may not be able to hit max lifts, and may feel worse in training compared to the first part of your cycle. So, don’t judge the results of your training based on your performance in this phase alone. Decreased performance is a perfectly normal experience in the luteal phase of your cycle."

Menstrual Cycles Also Affect Your Rest & Recovery

It's not just your performance at a workout that seems to be tied to menstrual cycles, either. It should also be part of how you look at your rest and recovery time. "You might want to schedule your rest days during your luteal phase," says Hardy, as the luteal phase means the body is lower in energy anyway. "Also, if you want to take time off from training for vacation, your luteal phase is a great time to take it in order to reduce impact on your strength goals."

A lot of research remains to be done on the intersections between the menstrual cycle in women and exercise and endurance. In many ways, menstrual science is really just getting started. But whether you're a serious athlete or just prone to a jog a few times a week, factoring in your individual menstrual cycle stage might help you understand your body better and tailor your workout needs and expectations in a more personal way.